(1) Field of the Invention
The invention relates to an echo canceler, particularly for use in a data modem for simultaneous two-way transmission of data signals on two-wire connections.
(2) Description of the Prior Art
Modern transmission systems usually consist of a combination of two and four-wire connections. Four-wire connections consists of two one-way paths, namely a one-way transmit path and a one-way receive path. A two-wire connection is formed by a two-way path on which signals can be simultaneously transmitted in mutually opposite directions in one and the same frequency band. These different paths are interconnected by means of a hybrid.
As known, a hybrid is formed by a four-port circuit. A first port, the so-called transmit port, is connected to the one-way transmit path, a second port, the so-called receive port, is connected to the one-way receive path, a third port, the so-called cable port is connected to the two-way path and a balancing network is connected to the fourth port, the so-called balancing port. This balancing network is intended to match the hybrid to the cable impedance. With perfect adjustment of this balancing network, a signal in the transmit path will appear at the cable port, but not in the receive path. If, on the contrary, a signal is applied to the cable port on the two-way path, this signal will appear in the receive path but not in the transmit path.
As each two-way path contains one or more discontinuities from which a signal can reflect, a reflected version of the signal occurring in the transmit path may be expected in the receive path.
Owing to the various cable lengths and cable types the cable impedance is usually not precisely known so that the balancing network can never be perfectly adjusted. The result is that a portion of the signal occurring in the transmit path will also find its way into the receive path via the hybrid.
The portions of the signal present in the transmit path which occur in the receive path in the above-described manner will, as customary, be called, echo signal.
Such echo signals have a particularly bad influence on the quality of the signal in the receive path. In order to improve this quality, circuits have been designed to cancel the echo signals or to reduce, at least, their influence as much as possible. These circuits are known as echo cancelers.
As will be apparent from references 1, 2 and 3 of paragraph D an echo canceler usually comprises:
an adjustable signal processing device coupled to the one-way transmit path;
a difference producer connected to the one-way receive path for generating a residual signal indicating the difference between the signal occurring in the receive path and the signal produced by the signal processing device;
an adjustable device responsive to control signals for adjusting the signal processing device; and
a control signal generator for generating the said control signals and which is responsive to the said residual signal.
In response to a signal which occurs in the transmit path, the signal processing device generates a synthetic echo signal, whose shape corresponds as closely as possible with the echo signal to be expected. The degree of agreement between synthetic echo signal and real echo signal is determined by the setting of the signal processing device, which is catered for by the adjusting device. This adjusting device, to which the output signal of the control signal generator is applied, is constructed so that it is capable of deriving from the residual signal a signal which is a measure of the non-suppressed echo signal present in the residual signal, the so-called residual echo, and of adjusting the signal processing device such, by means of this signal in an iterative manner, such that a minimum value of the mean-square value of the residual echo is obtained.
As indicated in the references 1, 2 and 3 the control signal generator is usually formed by an analog-to-digital converter to which the residual signal is applied and which converts this residual signal into a digital signal. For this conversion the residual signal is first sampled with a suitable sampling frequency. When this echo canceler is used in a datum modem which is arranged for transmitting and receiving data signals which are formed by a sequence of data symbols occurring at a symbol rate 1/T the sampling frequency may also be made equal to 1/T (see reference 1).
The signal processing device is preferably in the form of a non-recursive digital filter, whose filter coefficients are determined by the adjusting device (see the references 1 and 3).
The adjusting device comprises a digital correlator which includes a multiplying device and an accumulator. Applied to this multiplying device are the input signal of the signal processing device and the control signal. The output signal of this multiplying device is applied to the accumulator. The content of this accumulator now constitutes the desired filter coefficient which is non-destructively read at the symbol rate 1/T.
Although these prior echo cancelers operated satisfactorily, a considerable increase in the quality of the residual signal appears to be possible, namely by constructing the signal processing device as an interpolating digital filter (see the references 4 and 5) having an interpolation factor M. M will usually represent a positive integer. The consequence thereof is that also the sampling pulses applied to the analog-to-digital converter must occur at a frequency M/T.